The present invention relates to improvements in the art of photo-keratometry and more particularly to the use of television techniques to ascertain the contour of the cornea. A keratometer is an instrument for determining the shape of the corneal surface which generally uses a placido or other illuminated target that is viewed by the patient. The reflection of a placido or other target by the tear film on the anterior surface of the cornea is analyzed to determine the surface contour of the eye.
The technique in modern form dates from the early thirties when the Zeiss optical company of Germany introduced a "Photo Keratoscope". In general, the art has required the image reflected by the eye to be photographed and the image on the film measured in a second step to derive the data from which the contour map is generated.
Recent improvements have been in the area of automating this photogrammetric analysis by re-imaging the photograph with television apparatus and digital signal conversion. After digitization, computer analysis of the resultant information is performed with conventional image analysis algorithms. This type of data analysis is computer intensive and the image formed by the television system contains a large amount of redundant and extraneous information. For adequate resolution the sampling rate must exceed the data frequency by at least three to one, thus generating a huge number of data points for mathematical analysis. Consequently the systems are costly, complex, slow and often lack real resolution in the image analysis.
Other means have been used for clinical measurements such as direct casting of the eye surface in plastic or wax and coating the cornea with talcum powder and projecting a grid on this surface for photogrammetric analysis. At present, the clinical standard is the Bausch and Lomb Keratometer, which is sold commercially. The Bausch and Lomb Keratometer only measures the average of the corneal radius in two meridians of the central 3 mm "cap" of the cornea. The standard technology does not provide total surface topography of the cornea and thus is inadequate for many diagnostically significant abnormalities or the needs of some surgical procedures. In addition, the prior art technique is cumbersome and involves great potential for error.
The initial development in keratometry came from Gullstrand in 1896. Gullstrand disclosed the foundation for the current technology but his apparatus had no provision to compensate for aberrations in the optical system other than limiting the photographic coverage of the cornea to a 4 mm area. As a result, multiple exposures and calculations were necessary to map the corneal surface.
Much of the modern technique was developed by Amsler in 1930 and embodied in his "Photo-Keratoscope" which also required measurement and calculation as a separate step to derive the corneal shape data.
As noted supra, the standard instrument which is in most common use for central optical zone shape measurement is the Bausch and Lomb Keratometer. Several companies offer similar devices with similar principles of operation. In these devices two perpendicular axes are used to create an image of a small portion of the anterior surface of the cornea. The user is required to operate several controls to bring Mire images reflected from the two perpendicular axes simultaneously into focus and alignment. In addition, the operator manually records the data obtained. Other instruments are also available, such as the Haag-Streit Javal Schiotz device which measures only one axis at a time, but is slightly easier to use and tends to be more accurate in practice than the Bausch and Lomb system. In addition there exists a photo system made by International Diagnostic Instrument Limited under the trademark "CORNEASCOPE" (and a similar system made by Nidek in Japan), as well as autokeratometers by several manufacturers. The CORNEASCOPE produces Polaroid photographs of the reflection of a placido disc and requires a second instrument separate from the camera assembly to analyze the data. This system is fairly accurate, but expensive and tedious to use. The autokeratometers all are limited to a single zone of approximately 3 mm diameter and, in cases where the magnitude of the astigmatism is low, are inaccurate in their assessment of axes of astigmatism. Also available are three computer-direct systems which use conventional image analysis algorithms in conjunction with a mini-computer. These are the previously mentioned Computed Anatomy system, the ECT-100, recently introduced into the market by Optimed of Alphareta, Ga., and a system using light emitting diodes disposed in concentric rings built by Zeiss of Germany. The placido disc-photo technique is superior to the Bausch and Lomb Keratometer because of the much greater amount of information about zonal shape which may be obtained from the placido reflex as opposed to the mires of the Keratometer.
A number of patents have been issued that relate to keratometers. U.S. Pat. No. 3,797,921 discloses the use of a camera to record the placido reflex from a patients eye. From this photograph, the radius of surface curvature of the cornea is determined at several points and calculated using a complex computer system. The use of a ground glass focusing screen with the small aperture of the optical system and large linear magnification makes use difficult and requires a darkened room for operation.
U.S. Pat. No. 4,440,477 discloses a method and device for measuring the corneal surface, comprising a slit lamp for illuminating the corneal surface, a camera for recording the reflection from the corneal surface, and a processor to calculate the image distance and the radius of curvature of the eye. The operation of the processor is not detailed in U.S. Pat. No. 4,440,477.
A more recent entry into the market is the "Corneal Modeling System" manufactured by Computed Anatomy Incorporated of New York which uses a scanning laser in conjunction with a "frame grabber" to digitize and store for conventional image analysis the pictorial data. The placido is in cylindrical form which requires large depth of focus of the imaging system and, consequently, sophisticated focus determining apparatus to assure accurate image evaluation. This system is said to produce corneal thickness data as well as the surface contour but is very expensive and does not lend itself to clinical applications which are increasingly cost driven.
All the prior art systems are both expensive and difficult to use. The prior art devices all have a significant potential for error, due to complexity of the calculation, the imaging of the corneal surface and the difficulty in operating these systems. The traditional approach to photo-keratometry has been very software intensive and thus quite costly. In addition, the digitized image occupies a large portion of memory. The digitized image must occupy only a portion of the available memory in any computer system if there is to be the capacity to act upon the image information. For example, if the image is composed of some 400 active lines, each line must be subdivided into elements for storage. If equal horizontal and vertical resolution are required, then 533 elements must be converted and stored for each video line (given the standard 4:3 aspect ratio). This would result in 213,000 elements to be stored (400.times.533). In addition, the program, which involves complex computations, would use a large amount of memory for program and variable storage. From this it follows that some other system must be employed to produce a functional instrument within clinical cost constraints and operable by unsophisticated users. These and other criteria require that a departure from the traditional techniques for Keratometry and image analysis be employed.